Files can now have a relative path in the tar that gets sent to the build agent. This solves the case where a Dockerfile contains "ONBUILD ADD target/x /y". Before, the build could not find "target/x" because the file in the tar did not contain a relative path. This way, both "docker build ." and "mvn package" work with the same Dockerfile.

- DockerFile as a separate configuration attribute
- Artifacts ("files") can now have a destination in the tar

Better error message when one of the files is not present
no gzip
artefact as directory
Changed the snapshot version to 3.0, because this release will break compatibility
made the destination code a bit more readable using Optional.
Re-enabled Integration tests and updates CHANGELOG & README.
Merge branch 'remmelt-feature/directory_as_artefact'